util: add base class for transactional context managers We have at least three types with a close() and a release() method where the close() method is supposed to be called on success and the release() method is supposed to be called last, whether successful or not. Two of them (transaction and dirstateguard) already have identical implementations of __enter__ and __exit__. Let's extract a base class for this, so we reuse the code and so the third type (transactionmanager) can also be used as a context manager. Differential Revision: https://phab.mercurial-scm.org/D392